Cities have been acknowledged as the critical actors in the context of climate change as they account for nearly 70% of the global CO2 emissions. Mayors and municipalities understand the urgency needed with the climate action and are acting at the forefront by engaging in transnational initiatives to support emissions reductions. The steady emergence of transnational municipal networks such as C40 or ICLEI is an indication of a paradigm shift in climate change governance. These TMNs have started to become diplomatic actors themselves and fostering climate action within the network. Hence, a mixed-method research design entailing both the quantitative and qualitative analysis is chosen to single out the impact of network membership on climate mitigation. The quantitative study pertains to a multiple regression analysis of factors that are expected to covariate with the CO2 emissions. Data from over 600 OECD cities were analysed. Although, the main hypothesis - i.e., that TMNs have a significant positive impact on CO2 emissions could not be verified. The qualitative analysis was performed after the statistical analysis and two case studies were compared: Bogota and Copenhagen. The results showed that C40 membership cannot be associated with the outcome i.e., - change in emissions owing to its limited influence in the implementation phase. The triangulation of results showed no causality between network membership and decrease in CO2 emissions in the OECD cities for the period of 2001-2019. The policy advice is highly relevant for both local and national governments, to leverage the full potential of TMNs. Finally, this analysis is the first approximation to measure the 'impact' of a climate network, hence recommendations are provided for future avenues to be explored to substantiate and validate my findings.